1. Field of Use
The present invention relates generally to data processing systems and more specifically to the recovery of clock and data information written on magnetic media such as is coated on the surface of a disk or diskette where the timing of the information received varies from the expected timing.
2. Description of the Prior Art
The need for higher density recording of digital information on a magnetic medium has fostered the development of numerous encoding schemes including both phase and frequency encoding. With increased data processing speeds placing greater emphasis on high density magnetic recording, industry has resorted to frequency modulation (FM) and modified frequency modulation (MFM) binary information streams. The FM and MFM information streams are generally referred to as single density and double density encoded information, respectively.
U.S. Pat. No. 3,108,261 issued to Miller, entitled "Recording And/Or Reproducing System" describes a form of MFM recording using a considerable amount of circuitry.
FM or single density read recovery is disclosed in U.S. Pat. No. 4,034,348 to Rathbun wherein feedback loops are used to generate timing strobes for detecting both clock and data bits. Further, MFM or double density encoding has been disclosed in a Control Data Corporation project note entitled "MFM Double Density FDD System", dated Nov. 16, 1976. The insertion of an illegal logic zero clock signal in the address mark field to provide an address mark and the use of delay lines to accommodate a read recovery also are disclosed.
The read recovery systems in general have been comprised of frequency sensitive devices such as delay lines, variable frequency oscillators (VFOs), phase lock oscillators (PLOs) and feedback loops for generating timing strobes. The delay lines have exhibited a limited accuracy which is unacceptable in contemporary data processing systems. The VFO and PLO systems must be tuned to the rate at which information is to be received from a storage medium. When the VFO or PLO tuned frequency drifts or the system architecture is changed, a retuning is required to prevent read errors. The accuracy of VFO and PLO systems in addition is affected by the occurrence of write splices which may be present in the information stream as a result of an updating of data records on the storage medium. The delay lines, VFOs and PLOs further are not compatible with integrated circuit implementation. Those read recovery systems using the feedback loop method of generating timing strobes are the more complex of the prior art systems. Because of the complexity, feedback loops have been limited to FM read recovery systems.
Another example of a VFO tuned system is U.S. Pat. No. 3,753,143 entitled "Phase Locked Oscillator for Integer Pulse Rates", describes a means of locking a variable controlled oscillator to the bit rate of the incoming train of data pulses.
U.S. Pat. No. 4,212,038 which issued July 8, 1980 and entitled "Double Density Read Recovery" is directed to a logic system for converting MFM encoded information to a Non-Return to Zero (NRZ) information stream without the use of delay lines, VFOs, PLOs or timing strobe feedback loops. Instead, logic ONE bits occurring in the information stream are applied serially through an input shift register. Outputs of the input shift register are sampled by a multiplexer in response to a control signal indicating the occurrence of clock bits in the MFM information stream. Timing strobes for controlling the operation of the logic read recovery system are thereby generated.
The system described in U.S. Pat. No. 4,212,038 had the disadvantage of requiring a considerable number of integrated circuit elements to provide the read recovery of MFM digital signals. To these logic circuits were added a considerable number of integrated elements to provide the read recovery of FM digital signals.
It should be understood that the references cited herein are those which the applicants are aware and are presented to acquaint the reader with the level of skill in the art and may not be the closest reference to the invention. No representation is made that any search has been conducted by the applicants.